Typically, biological and biometric infra-red sensors use silicon based PN-junction photo-detectors. The low energy, typically 2 E-19 J (1.24 eV), photons detected by the photo-detector penetrate deep into the photo-detector. This results in the generation of carriers in the photo-detector at a significant distance, for example 10-50 μm from the desired detection region. Consequently, a significant fraction of the carriers generated by the infra-red photons do not contribute to the sensed photo-current. This leads to a low sensitivity limit of the photo-detector.
Furthermore, amplification of the sensed photo-current, as a means of overcoming the low sensitivity limit, is hindered due to the effect of photo-diode leakage current, the so called “dark current” and other effects such as shot noise.
Attempts have been made to overcome the inherent lack of IR sensitivity of PN-junctions by the use of Positive-Intrinsic-Negative (PIN) photodiodes. The structure of a PIN photodiode produces a deep junction region that is more sensitive to infra-red radiation. However, PIN diodes are not compatible with modern deep sub-micron CMOS technologies. This is because the junction depth of the PIN photodiode must be of the order of 50 μm for optimal efficiency at a wavelength of 950 nm.
In addition to the above, the sensor may not be completely covered by a test subject. This allows stray light, for example ambient visible and infra-red (IR) radiation, to impinge upon the sensor. The ambient visible and infra-red radiation decreases the signal-to-noise ratio associated with the sensor, further reducing the sensor's sensitivity. This problem is exacerbated when the sensor has a low initial sensitivity.
FIG. 1 shows an IR detection system in which a lock-in detection circuit has been used to subtract a background level of photodiode current from a signal current. A controller 102 synchronizes the operation of a first switch 104, used to operate an IR LED 106, and a second switch 107, used to switch between positive and negative unity gain amplifiers 108, 110.
The measured signal current comprises not only the current resulting from the signal caused by photons received from the LED 106 (as desired), but also a background level current. This background level current comprises the sum of the currents which result from the receiver offset voltage, the background signal due to ambient light and the photodiode leakage current.
The circuit of FIG. 1 removes these background signals by switching to the negative unity gain amplifier when the IR LED 107 is switched off, during which time only background signals are measured. The output generated from the low pass filter 112 therefore comprises the difference between the original measured current and the background level, which is the desired current resultant from the signal received from the LED 106.
Unfortunately, this circuit is not compatible with production in a CMOS integrated circuit, and is therefore of limited use, practically. This is because of the resistor 114 that is used in conjunction with operational amplifier 116. The gain of the operational amplifier 116 is proportional to the resistance of the resistor 114 and hence a large gain requires a large resistor 114. Large resistors require large volumes of silicon to produce and are expensive. Furthermore, resistors in general are best avoided, where possible, in integrated circuits such as these. Finally, as the unity gain amplifiers 108,110 have their outputs averaged by the low pass filter 112, a stringent requirement of matching is placed upon the amplifiers 108,110. This is complex to achieve.